Aspirin has a proven ability to prevent cancer which should be factored into new patient guidelines, according to the British lead author of a major new study.

Professor Peter Rothwell believes the evidence shows aspirin's anti-cancer benefits may be bigger than its protective effect on the heart and arteries.

His research, published in two Lancet journals, suggests that low daily doses of the painkiller may not only prevent cancer occurring, but also slow its progress.

One study showed that taking aspirin reduced the risk of developing cancer by about a quarter after just three years. From five years onwards, it cut the risk of dying from cancer by 37%.

Another in a series of three papers showed that aspirin reduced the chances of cancer spreading instead of staying in one place by almost 50%.

The deadly spread, or metastasis, of tumours to organs such as the liver and brain is usually what kills cancer patients.

Many people take a low 75 milligram dose of aspirin each day to guard against heart attacks and strokes.

Experts advise against this for "healthy" individuals at no special risk of heart and artery disease because of the possible long-term side effects of aspirin. The drug, which prevents blood clotting, can increase the likelihood of internal bleeding in the stomach, intestines and brain.

In some cases, such as pregnant women at risk of high blood pressure, the benefits of taking aspirin are said to outweigh the risks. However, to date cancer has not been part of this calculation.

Prof Rothwell, from Oxford University, said: "It's certainly time to add prevention of cancer into the analysis of the balance of risk and benefits of aspirin."